1. Field of the Invention
The invention is concerned with plastic containers produced by blow molding. Of particular interest is blow molding as carried out over the lower range of permissible temperature so as to result in biaxial orientation.
2. Description of the Prior Art
Plastic containers, that is, containers constructed primarily of substituted or unsubstituted hydrocarbons have, for some time, been in prevalent use. Increased popularity, as compared with inorganic glasses, is due to a variety of factors--inter alia, improved impact resistance, reduced weight, and, perhaps most significantly, reduced cost.
As would be expected, procedures for producing so popular a commodity have been in continuing transition. What started as a simple operation, analogous to that used for inorganic glass has evolved through a variety of stages. Today, most containers are produced by procedures which involve a first formation of a preform, sometimes denoted a parison, followed by expansion of the preform to produce the final container. Preforms are commonly produced by any of the various molding operations with extrusion and injection being predominant. Expansion is commonly by means of gas pressure with final form being determined by a mold specifically designed for this operation.
A most sophisticated form of blow molding is now becoming commercially significant. It differs from earlier procedures in that the parison is first stretched and then blown while its temperature is much lower than with conventional processes--generally, within the 100.degree. F. range above T.sub.c (the temperature at which the plastic material passes from the glass phase to the rubber phase). This procedure, which is known as stretch-blow molding, or orientation blow molding, is not only replacing more conventional procedures, but is opening up new markets. Advantages of the new procedures are all related to biaxial orientation of the polymer material. Regardless of how the parison is initially formed--e.g., whether by extrusion or injection, regardless of whether the procedure is two-stage or in-line, the parison, while at a temperature insufficient to permit free plastic flow is expanded biaxially to conform with the blow mold. The temperature of operation generally within the 100.degree. F. range immediately above T.sub.c is such that expansion introduces true strain into the material. The strain translated into a definable polymer orientation results in a number of attributes.
Alignment of polymer molecules results in increased tensile strength, as well as increased clarity, increased impact strength, and reduced creep. A vast market for carbonated soft drink containers is a direct result of significantly improved gas barrier properties.
Suitable container materials are substituted and unsubstituted thermoplastic hydrocarbons. Commonly used materials at this time include acrylonitrile, polyethylene terepthalate, and polypropylene. The described procedures are well known--descriptions are included in standard reference texts. See, for example, Modern Plastics Encyclopedia, Vol. 54, No. 10A, 1977-1978, McGraw-Hill Publishing Co. (e.g., sections on "Blow Molding" at page 230 et seq.; "Injection-Blow Molding" at page 232 et seq.; and "Stretch-Blow Molding" at page 233 et seq.). It is well known that the various types of stretch-blow molding procedures, as well as conventional injection blow molding procedures, have not been successfully adapted to the production of a container including an integral handle. Commercially, plastic containers with handles have been made exclusively by extrusion-blow molding procedures wherein a large diameter parison is pinched in such a way as to define a handle area which is subsequently blown with the rest of the container to its final dimensions. This process depends on the use of high plastic temperatures to provide satisfactory fusion of the plastic in the blow mold when it is pinched prior to expansion with the pressurizing medium. It also depends on the use of large diameter parisons and forming the neck finish in the blow mold. A further limiting feature is that such handles are hollow and interconnected with the cavity of the container so that the handle must also act to contain product. This and other design limitations preclude consideration of such processes for certain important markets.
The problem of producing plastic containers with handles using stretch-blow processes derive from the nature of the process and the condition of the plastic at the time it is biaxially oriented. It is not possible to produce handles by the aforementioned parison pinch-off technique because the plastic temperature required for suitable molecular orientation is much too low to permit adequate fusion of the plastic. To form the handle and neck finish first at elevated temperatures and then cool to biorientation temperatures before stretching and blowing would yield handles and substantial other unoriented portions of the container with inferior containment and other properties. Additionally, the prime current market for bioriented containers is for soft drinks where its success depends on the use of a bottle shape optimally designed for pressurized use. Such designs are not possible in the processes heretofore considered.
And so the common assessment of the market place has been that stretch-blow molding processes are incompatible with the production of integral handles and attempts to overcome this problem have generally taken the form of mechanical fixtures. A variety of discrete handle configurations have been attached by means of bands and the like. Such attachments have been suitable for carrying but not for pouring with a one-handed grip.
An approach described in French Pat. No. 1,192,475 has apparently not found commercial use. The procedure described in conjunction with FIGS. 12 and 13 of that reference as applied to conventional (not stretch) blow molding makes use of a completely formed return handle attached at both upper and lower extremities to a parison portion not subjected to blow molding. While permitting undisturbed blowing in the bottom portion of the parison, this approach is undesirable because an excessive portion of the length of the parison cannot be blown resulting in inefficient material utilization, container shape and size restrictions even when a small, one-finger, jug-style handle is contemplated. Additionally, such an approach when translated to stretch-blow processes leaves substantial portions of the container between the extremities of the handle in a non-oriented, and therefore inferior, state.